1. Field of the Invention
The present invention relates to a spatial optical communication apparatus for performing communication by transmitting an optical signal with a light beam through free space between spaced opposed locations, and particularly to an apparatus having a function of correcting the deviation of an optical axis of a light beam due to the angular shift of the apparatus.
2. Related Background Art
In general, with a spatial optical communication apparatus for performing communication by transmitting a light beam through free space, it is necessary to use a narrow light beam whose divergent angle is made as small as possible such that the power of light can be effectively transmitted. However, when the light beam is narrowed, the light beam is liable to deviate from a partner apparatus due to the angular variation caused by vibrations of a building or an installation mount resultant from wind pressure, distortion resultant from temperature fluctuation, or the like. Stable communication is hence difficult to achieve.
Accordingly, such an apparatus as illustrated in FIG. 4 has been proposed. The apparatus has an optical-axis deviation correcting function that the light beam is always directed to the partner apparatus by correcting the angular change even if the angle of the apparatus is varied.
FIG. 4 illustrates one of a pair of opposed apparatuses. In FIG. 4, reference numeral 10 denotes an optical system for performing transmission and reception of a light beam. A transmission optical signal to the partner apparatus is emitted from a light emission element 21 such as a semiconductor laser. Light from the semiconductor laser is polarized, and its polarization direction is set parallel to the sheet of FIG. 4. This polarized light is reflected toward a light transmission and reception lens 23 by a polarization beam splitter 22. The light is converted into a substantially parallel light beam 24 with a small divergence by the lens 23, and transmitted toward the partner apparatus.
Light from the partner apparatus is reversely guided along the same optical axis as that of the transmission optical signal from the subject apparatus, and the light thus passes the light transmission and reception lens 23, and enters the polarization beam splitter 22. This reception light from the partner apparatus, however, transmits through the polarization beam splitter 22 since its polarization direction is set orthogonal to that of the transmission light (perpendicular to the sheet of FIG. 4). The reception light thus enters a beam splitter 25. Most part of the reception light is reflected by the beam splitter 25, and enters a light receiving element 26 for detecting an optical signal. A communication signal is thus detected. On the other hand, part of the reception light transmits through the beam splitter 25, and enters a light position detecting element 27.
The light position detecting element 27 is a four-division photodiode as illustrated in FIG. 5, for example. FIG. 5 illustrates a situation in which a light spot 42 impinges on four divided photodiodes 27a to 27d. The position of the light spot 42 can be obtained by comparing outputs of the four photodiodes 27a to 27d with each other. A signal from the position detecting device 27 is arithmetically and logically processed as angle correction information by a control circuit 28. A driving signal is thus supplied to a drive circuit 29 for the optical system 10. Driving mechanisms 30 and 31 for vertical and horizontal directions are moved by the drive circuit 29, and the angle of the optical system 10 is thus driven and controlled such that the light spot 42 can approach a central portion of the light position detecting element 27 to equalize all outputs of the four photodiodes 27a to 27d with each other.
In the optical system 10, positions of the light position detecting element 27, the light emission element 21, and the light receiving element 26 for detecting the optical signal are adjusted such that all their optical axes coincide with each other. Accordingly, when the light spot 42 impinges on the central portion of the light position detecting element 27, the light also impinges on the central portion of the light receiving element 26 for detecting the optical signal, and a central portion of light from the light emission element 21 is emitted toward the partner apparatus.
The optical-axis deviation correction is thus executed such that the transmission light is always directed toward the direction of the reception light, i.e., the partner apparatus.
Further, there has also been proposed an apparatus that has a light deflecting mechanism in the optical system 10 in lieu of the mechanism for correcting the optical-axis deviation correction by driving the optical system itself. For example, in such a mechanism as illustrated in FIG. 6, angles of deflection mirrors 32 and 33 for horizontal and vertical directions are driven by the drive circuit 29, and the optical-axis deviation correction is performed by deflecting light in the lens barrel.
In the above two conventional apparatuses, the optical-axis deviation due to the angular fluctuation of the apparatus is corrected such that communication can be stably achieved even when the divergent angle of the light beam is decreased. Each apparatus, however, has its advantage and disadvantage. Specifically, in a system which includes the driving mechanism provided outside the optical system, and changes the angle of the entire optical system as illustrated in FIG. 4, the variable angle range can be freely widened to cope with a wide angular variation, but it is difficult to rapidly change the angle and to respond to a high-speed angular variation since a relatively heavy mechanism needs to be driven.
On the other hand, in a system which includes the light deflection mechanism provided inside the optical system as illustrated in FIG. 6, a high-speed angular variation can be handled since it is relatively easy to rapidly change the angle of light, but it is difficult to increase the variable angle range since the size of the optical system is limited and the light beam is hence likely to deviate from the optical system if the angle is largely changed.